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18.1 Studying Viruses and Prokaryotes 4c Key Concept Infections can be caused in several ways. MAIN IDEAS VOCABULARY virus pathogen viroid prion 4C compare the structures of viruses to cells, describe viral reproduction, and describe the role of viruses in causing diseases such as human immunodeficiency virus (HIV) and influenza Viruses, bacteria, viroids, and prions can all cause infection. Connect to Your World Bacteria are everywhere, including in and on your own body—such as the bacteria that live in our digestive tracts. The relationship between you and the microorganisms in your body is usually mutually beneficial. Under certain conditions, however, normally harmless microorganisms can cause disease, and some types of microorganisms are particularly nasty—they always make you sick. MAIN IDEA 4c FIGURE 1.1 Prokaryotes, such as this Escherichia coli bacterium, are single cells that have all of the characteristics of living things. (colored TEM; magnification 60003) 528 Unit 6: Classification and Diversity You are probably familiar with the terms virus and bacteria, but you may not know exactly what they are. A virus is an infectious particle made only of a strand of DNA or RNA surrounded by a protein coat. Bacteria, on the other hand, are one-celled microorganisms that can also cause infection. Any living organism or particle that can cause an infectious disease is called an infectious agent, or pathogen. You have learned that all living things share certain key characteristics: the abilities to reproduce, to use nutrients and energy, to grow and develop, and to respond to their environments. They also contain genetic material that carries the code of life. Prokaryotes—such as the bacterium shown in FIGURE 1.1—are clearly living things, since they have each of the traits of life. But are viruses living things? Like living cells, viruses respond to their environment. Viruses have genes and can reproduce. Unlike cells, however, viruses cannot reproduce on their own. Instead, they need living cells to help them reproduce and make proteins. Viruses are also much smaller than most cells, as you can see in FIGURE 1.2. While viruses have key traits similar to living cells, they also have many differences. In fact, viruses are not even given a place in the Linnaean system of biological classification. A viroid has even less in common with living things than do viruses. Viroids are infectious particles that cause disease in plants. Viroids are made of single-stranded RNA without a protein coat. They are passed through seeds or pollen. Viroids have had a major economic impact on agriculture because they can stunt the growth of plants. ©Dr. Linda Stannard, UCT/Photo Researchers, Inc. Viruses, bacteria, viroids, and prions can all cause infection. FIGURE 1.2 Relative Sizes of Cells and Infectious Particles Although eukaryotic and prokaryotic cells can be microscopic, they are large in comparison to viruses, viroids, and prions. eukaryotic cells 10,000–100,000 nm 1 nanometer (nm) = one billionth of a meter 100 nm viroids 5–150 nm viruses 50–200 nm prokaryotic cells 200–10,000 nm prion 2–10 nm Infer Why are viroids and prions sometimes called subviral particles? At the boundary between living and nonliving, perhaps the strangest entity of all is the prion. A prion (PREE-ahn) is an infectious particle made only of proteins that can cause other proteins to fold incorrectly. When proteins misfold, the protein will not work properly. Prions are unusual in that they are infectious yet have no genetic material. They play a part in certain diseases of the brain such as mad cow disease, known to scientists as bovine spongiform encephalopathy, or BSE. Humans may become infected with BSE when they eat meat from animals that are infected. Food safety laws in the United States, however, try to reduce the risk of infection. Creutzfeld-Jakob (KROYTS-fehlt YAH-kawp) disease (CJD), another brain disease that affects humans, is also associated with prions. Prion diseases can incubate for a long time with no effect on their host. However, once symptoms appear, they worsen quickly and are always fatal, because the body has no immune response against a protein. R E A D I NG T O O L B o x TAKING NOTES Use a two-column chart to take notes on viruses, viroids, and prions. Main Idea Detail Virus Viroid Prion Synthesize Why are viruses, viroids, and prions not included in the Linnaean system of biological classification? Self-check Online 18.1 Formative Assessment Reviewing Main Ideas 1. What are the main differences between living cells and viruses? 4c 2. Viruses, viroids, prions, and some bacteria can all be considered pathogens. What do all pathogens have in common? Critical thinking 3. Infer Prions were not widely known to be infectious agents until the 1980s. Give two reasons why this might be so. 4. Apply An RNA-based disease spreads through pollen. Is it likely due to a virus, viroid, or prion? Explain. HMDScience.com GO ONLINE CONNECT TO Medicine 5. To multiply, viruses must take over the functions of the cells they infect. Why does this make it difficult to make effective antiviral drugs? 4c Chapter 18: Viruses and Prokaryotes 529